The present invention relates to improvements in the self-tapping screw and the thread rolling die used for manufacturing the same more particularly, the present invention is directed to the improvements in the self-tapping screw which is rolled such that a plurality of cutting screw threads performing the cutting function are shaped on the top end of the screw and the thread rolling die for making the same.
Conventionally, this kind of self-tapping screw has been widely used such that a hole is made on a metal plate by means of the cutting section located on the top end of the screw, thereby forming a tapped hole, while fastening the screw itself. In particular, the self-tapping screw with the cutting screw thread rolled on its top end is advantageous in simplification of the machining process in comparison with other kinds of self-tapping screws, because the rolling of fastening screw thread to be provided on the base end of the screw can be performed at the same time as that of the cutting screw thread.
In this connection, various kinds of improvements are submitted, when making this kind of self-tapping screw, in order to attain an advancement in the cutting performance of said screw. It is already well-known that this advancement in the cutting performance requires the cutting screw thread to be extruded to a large extent. Nevertheless, if the cutting screw thread is extremely extruded, it gives rise to a problem disadvantageous in performing so-called relieving of the cutting screw thread.
With regard to such a conventional self-tapping screw as shown in FIG. 1 (Japanese Patent Provisional Publication No. 56-160414), although the external diameter (h1) of the cutting screw threads 1 and 1 is extruded up to being approximately equal to the diameter of the bottom of the thread (h2) of the screw's section proper, if the former diameter is to be measured near the top end of the screw, a problem of wearing the cutting screw thread due to frictional heat is incurred, because, on the cross section perpendicular to the center of shank 2, as shown in FIG. 2, the distance from the center of the shank 2 to the resistance flank 3 becomes equal to that from the center of the shank 2 to the top of the screw thread 4. Thus the resistance flank 3 is placed in frictional contact with the tapped hole made in an article to be cut during the cutting work so that the turning torque is increased.
On the other hand, as shown in FIG. 3, a majority of grooves shaping cutting screw threads 5 are juxtaposed on the lower part of the thread rolling die for making the self-tapping screw. Similarly to the groove shaping the fastening screw thread 6, however, the conventional groove shaping the cutting screw thread 5, which is provided on the above kind of die, has adapted each of the angles formed between the face shaping the resistance flank 7 and the face shaping the cutting flank 8 to be always constant. That is partly because it has been traditionally thought that a series of the screw threads in question might be occasionally changed in their height, but there is no probabiity of changing their angle, and partly because it has been believed that machining the thread rolling die such that both the angles between the face shaping the resistance flank and the face shaping the screw thread and between the face shaping cutting flank and the face shaping the screw thread are gradually changed only with extreme difficulty. Nevertheless, even if the conventional thread rolling die is successful in making the self-tapping screw the screw thread of which is adapted to be extruded to a large extent, as stated above, the problem of facing the inability in advantageously performing the relieving of the screw thread arises.